I just had a random theory about Deckard in 2049 (certainly already proposed by others).
Maybe old man Deckard is a clone or replicant of 2019 Deckard, perhaps created by Neander Wallace (Leto) to usher Officer K (Gosling) along whatever journey of self-discovery he's laid out for him.
That way, Ridley can still insist that 2019 Deckard was a replicant who expired, and thus Tyrell Corp or Wallace's company would have later preserved his memories. Others could infer that 2019 Deckard was a human who was killed or died by other means ("We were being hunted!"), but again his memories and DNA were stored to later create an old man Deckard.